Limestone

This large sinkhole formed when the ground subsided into an underlying cave system in the Carboniferous Limestone, following heavy rain in 1968. GB Cave, near Cheddar, Somerset.

Karst is most often seen in limestone, a rock made up mainly of calcium carbonate. In Great Britain, the best developed karst landscapes and the longest cave systems are associated with the Carboniferous Limestone. This rock forms the karst landscapes of the Mendip Hills, the Yorkshire Dales and the Derbyshire Peak District.

However, karst is also developed in other limestones, for example the Devonian Limestones in South Devon. The major issues associated with these karst areas are water supply protection, geological conservation and engineering hazards. Subsidence associated with sinkhole formation does occur, but as much of the Carboniferous Limestone in the UK is in rural upland areas, there is often little impact on property and infrastructure. However, subsidence hollows are often sites for illegal tipping of farm and other refuse or waste which can cause rapid contamination of the groundwater and local drinking water supplies.

Cheddar Risings, Cheddar Gorge. A prolific cave-fed spring from the Carboniferous Limestone, and an important public water supply. Cave divers have followed the underground river upstream for several hundred metres.

Above Malham Cove, the Great Scar Limestone forms a spectacular limestone pavement with grykes (fissures) formed by the solvent action of rainwater on joints in the limestone.